Cesium vapor interacting with a tightly focused, intense midinfrared $\left(3.4\mu \mathrm{m}\right)$ source produces high harmonic radiation in the visible∕UV spectral range. The measured yields of harmonic orders 9–17 are found to yield good quantitative agreement with numerical simulations that include both the single-atom and the macroscopic response. The $5\text{th}–9\text{th}$ harmonic orders are generated with sufficient pulse energies $(\sim 100\text{pJ})$ for direct temporal measurements using an autocorrelation method and when correlated with bandwidth measurements are found not to be transform limited. A blueshift connected to a strong time-dependent ionization appears to be the cause of this spectral broadening.

• Received 13 November 2003

DOI:https://doi.org/10.1103/PhysRevA.69.033807